The stimulation of the plasma membrane (PM) H+-ATPase by boric acid was studied on a microsomal fraction (MF) obtained from ungerminated, boron-dependent pollen grains of Lilium longiflorum Thunb. which usually need boron for germination and tube growth. ATP hydrolysis and H+ transport activity increased by 14 and 18%, respectively, after addition of 2-4 mM boric acid. The optimum of boron stimulation was at pH 6.5-8.5 for ATP hydrolysis and at pH 6.5-7.5 for H+ transport. No boron stimulation was detected when vanadate was added to the MF, whereas an increase of 10-20% in ATP hydrolysis and H+ transport was still measured in the presence of inhibitors specific for V -type ATPase (nitrate and bafilomycin) and F-type ATPase (azide), respectively. A vanadate-sensitive increase in ATP hydrolysis activity was also observed in partially permeabilized vesicles (0.001%[w/v] Triton X-100) suggesting a direct interaction between borate and the PM H+-ATPase rather than a weak acid-induced stimulation. Additionally, we measured the effect of boron on membrane voltage (Vm) of ungerminated pollen grains and observed small hyperpolarizations in 48% of all experiments. Exposing pollen grains to a more acidic pH of 4 caused a depolarization, followed in some experiments by a repolarization (21%). In the presence of 2 mM boron such hyperpolarizations, perhaps caused by an enhanced activity of the H+-ATPase, were measured in 58% of all tested pollen grains. The effects of boron on Vm may be reduced by additional stimulation of a K+ inward current of opposite direction to the H+-ATPase. All experiments indicate that boron stimulates an electrogenic transport system in the plasma membrane which is sensitive to vanadate and has a pH optimum around 7, i.e. the plasma membrane H+-ATPase. A boron-increased PM H+-ATPase activity in turn may stimulate germination and growth of pollen tubes.